Thin film growth using hyperthermal ions: a surface morphology study

Low-energy ion deposition and molecular beam epitaxy were used to systematically study the influence of the deposition energy on the growth of Co on Si(1 1 1). Co ions with a well-defined energy between 0.1 and 155 eV were deposited onto a 7 × 7 reconstructed Si surface, using sub- as well as supramonolayer coverage. For submonolayer coverage (0.3 ML), the initial periodicity degrades gradually with increasing ion energy: while for low energies (∼10 eV), 7 × 7 features remain clearly visible, these features disappear completely for higher deposition energies. In the supramonolayer (2 ML) region, we observe an increase in island density with increasing ion energy, which can be explained by the increasing amount of adatom–vacancy pairs and island breakup. Additionally, hyperthermal deposition yields a lower surface roughness compared to thermal deposition, with an optimal energy window for growing smooth films at ∼25 eV. This optimum can be attributed to enhanced layer-by-layer growth due to the extra energy provided by the incoming ion.